An intermediate shaft of a steering device for a vehicle includes a sliding shaft in which, for example, a male shaft and a cylindrical female shaft are connected to each other to be expandable/contractible along the axial direction through engagement of splines (spline fitting) or the like. An outer circumferential surface of the male shaft or an inner circumferential surface of the female shaft including the spline is coated with a resin coating layer in some cases in order to fill a clearance between these shafts to reduce rattling noise or reduce backlash of a steering wheel occurring in a steering operation (see, for example, Patent Reference 1).
As one of methods for forming a resin coating layer, a fluidized bed powder coating method is known. Since no organic solvent is used in the fluidized bed powder coating method, this method has an advantage of low environmental load.
In the fluidized bed powder coating method, a powder coating material containing a base resin corresponding to a raw material of a resin coating layer is first prepared, and in a fluidized bed, the powder coating material is placed in a floating and fluidized state by blowing air or the like.
Subsequently, a male shaft or a female shaft, having been heated to a temperature equal to or higher than the melting point of the base resin, is immersed in the floating and fluidized powder coating material, and thus, the powder coating material is deposited, and allowed to melt and flow, on the outer circumferential surface of the male shaft or the inner circumferential surface of the female shaft (hereinafter sometimes generically referred to as the “deposited surface”), and then is solidified when the resultant is cooled thereafter, resulting in forming a resin coating layer on the deposited surface.
As the base resin corresponding to a raw material of the powder coating material, polyamide 11, polyamide 12 or the like is suitably used because such a resin can form a resin coating layer excellent in flowability when melted, and excellent also in the sliding property and the like.